In most cases when such a circuit breaker is closed on a load placed in an AC network, transient current and/or voltage conditions are observed.
The amplitudes of such conditions depend on the nature of the load (line, reactance, transformer, bank of capacitors, . . . ) and also on the instant in the voltage period when connection actually takes place.
In order to minimize such transient conditions which give rise to voltage and/or current surges in the network, so-called "synchronous" closure controls have already been designed to close each pole of the circuit breaker in a manner that is controlled to coincide with an instant in the voltage period when the transient conditions are zero for voltage or for current. By way of example, in FIG. 1, applying voltage to a shunt reactance is optimum in terms of transient current conditions if voltage is applied at the instant marked t2 which coincides with an instant when the voltage is at an extremum Vmax: under such conditions, the current that is established in the load at instant t2 does not have any transient conditions and obeys the sinewave I=I.sub.max.multidot.sin .omega.(t-t2). This result can be obtained by applying two appropriate successive time delays starting from the instant marked t0 at which the control system receives the order to close a pole:
a time delay K1 measured from an instant when the voltage is zero; and PA1 a time delay K2 which is theoretically constant, corresponding to the time lapse required for closing the pole at an instant t2, as measured from instant t1. This time delay is associated essentially with the time required K2' mechanically to drive the contacts of the circuit breaker, and to the prestriking time for the electric arc K2" which is struck between the contacts of the pole (which time is always constant).
Successful synchronous closure of a circuit breaker pole therefore depends particularly on the duration K2' required for the circuit breaker to operate under hydraulic control remaining constant over time. By way of example, for a network operating at 60 Hz, and when closing on a reactance, the maximum acceptable current under transient conditions requires closure to take place at a maximum voltage value Vmax with precision that is better than or equal to 1 ms.
It has been observed that in hydraulically-controlled circuit breakers using "very cold weather" oils, the repetitiveness of the mechanical time K2' is reliable over the temperature range -40.degree. C. to +50.degree. C. with the required accuracy, providing switching operations are very frequent. However, if the circuit breaker has remained open-circuit for a long period of time (e.g. several months), experience shows that the mechanical time K2' required on first closure following said standby period is subject to an amount of error that is incompatible with synchronous closure. The origin of this error is associated with the phenomenon known as "gumming" that occurs in the synthetic rubber sealing rings that serve to provide sealing around the outlet shafts of the hydraulic control system. This difficulty is particularly significant in that the parameters that influence this error, in addition to the standby time, are numerous and poorly known.